Parsing iCal Data

by Robert Pratte
August 18, 2005

One of the attributes of a killer application is that it does something cool: it allows you to view or organize information in new and interesting ways, saves you time, or helps you win that auction bid. Yet one of the fundamental aggravations of applications in general is that they don't always work well together; typically, you cannot send your data to mixed-and-matched applications unless they were explicitly designed to allow this. One of the great strengths of a language such as Perl is its ability to overcome these differences and act as "glue." As long as you can figure out what the incoming data looks like, and how the outgoing data should look, it is very simple to share data between previously incompatible applications. By simply building a parser between the applications and creating input files for the target application from the former's data, you extend the usefulness of your tools. In a sense, you can create killer applications out of various mundane tools on your system.

A somewhat trivial example of this sort of creation is an application that converts iCalendar data into a directed graph, readable by an application such as GraphViz. This example seems so trivial that you might ask yourself why you would wish to do such a thing. The answer is perhaps equally trivial: aside from the challenge factor, the ability to convert data could provide an alternative (or complement) to Gantt charts in project documentation, map relationships between events, etc. Moreover, by providing a simple way to allow disparate applications to interoperate, you can cumulatively build suites of applications, hopefully allowing for unforeseen advantages in the future.

Returning to the example, say you would like to take an iCal calendar (Figure 1) and turn it into an interesting visualization (Figure 2). How would you do this? Such an ability to convert formats is one step in constructing that killer application.

Figure 1. An iCal calendar

Figure 2. An alternate visualization of the calendar data

Reading the iCalendar Format

RFC 2446 defines the iCalendar format, which Apple's iCal application uses. Each iCalendar file represents an individual calendar and contains at least one block of event data in key:value tuples, starting with a BEGIN:VEVENT tuple and ending with END:VEVENT. Here is an example (with indentation added for readability) of a small iCalendar file containing two events:

BEGIN:VCALENDAR
        CALSCALE:GREGORIAN
        PRODID:-//Apple Computer\, Inc//iCal 2.0//EN
        VERSION:2.0
        BEGIN:VEVENT
                LOCATION:San Francisco
                DTSTAMP:20050618T151130Z
                UID:BDF17182-CA21-4752-8D4F-40A4FE47C90D
                SEQUENCE:8
                URL;VALUE=URI:http://developer.apple.com/wwdc/
                DTSTART;VALUE=DATE:20050606
                SUMMARY:Apple WWDC
                DTEND;VALUE=DATE:20050612
                DESCRIPTION:Lots of sessions.
        END:VEVENT
        BEGIN:VEVENT
                DURATION:PT1H
                LOCATION:Home
                DTSTAMP:20050618T151543Z
                UID:5F88A0EC-AD21-428E-AAAD-005F1B1AB72E
                SEQUENCE:6
                DTSTART;TZID=America/Chicago:20050615T180000
                SUMMARY:Set up File Server
                DESCRIPTION:Music server for the kids.
        END:VEVENT
END:VCALENDAR

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